The present invention relates generally to gears. More particularly, it relates to gears, for example transmission gears, having a reduced surface roughness resulting in increased contact fatigue life, improved wear resistance and improved performance.
Many methods of finishing gear teeth are known. For example, in gear hobbing and shaving, a gear is rotated in mesh with a gear-like cutter tool. The gear-like tool has cutting edges that extend up and down the sides of the teeth parallel to the plane of rotation. This is accompanied by a relative traverse between the gear and the cutter in a plane parallel to the axis of the gear and the cutter. The resulting finished surface roughness is in the range of 40 to 80 micro-inches Ra, where Ra is the arithmetic mean roughness.
Another method of finishing the teeth of a gear is known as gear grinding. In gear grinding, the resulting finished surface roughness is typically 15 to 35 micro-inches Ra.
In another method of gear finishing, a honing operation is performed. Here, the gear is rotated in mesh with a gear-shaped hone. Portions of the hone at the said of the gear teeth are fabricated from a plastic material that is relatively hard yet highly resilient. The honing operation occurs by rotating the hone in mesh with the gear while providing a traverse stroke parallel to the axis of the gear. This distributes the finishing action evenly throughout each gear tooth. The resulting roughness is typically 15 to 35 micro-inches Ra. Fine grit honing may yield surface roughness as low as 12 to 13 micro-inches Ra.
However, none of these finishing methods can improve the surface finish below approximately 10-12 micro-inches without significantly increasing the cost and process time.
Polishing compounds used for preparing metal parts for electroplating have been available, using liquid polishing compounds containing fine abrasive particulate. For example, U.S. Pat. No. 4,491,500 teaches a physicochemical process for refining metal surfaces. The disclosed two-step process first utilizes a liquid chemical followed by a burnishing liquid. It further involves the development of a relatively soft coating on the surface being treated, followed by the physical removal of the soft coating and continuous repair. Rougher areas greater than 70 xcexcm are first leveled through some form of mechanical action. U.S. Pat. No. 4,818,333 describes a similar process, focusing on the composition of a high density burnishing media used in the process. These processes can result in finishes less than 3 micro-inches Ra. Thus, processes and chemicals are described for reducing surface roughness.
Chemical finishing techniques such as etching and bright dipping are also widely known in the art of electroplating preparation for the purpose of achieving an ultra-smooth and clean surface.
The main failure modes for gears are pitting or micropitting, wear and scuffing. When a gear and pinion interact, the gear teeth necessarily contact each other. Without lubrication, the teeth scratch against each other, scuff each other, wear down, pit and crack. Lubrication postpones the onset of these effects. Thus, the better the lubrication, the longer the life of the gear. Gears with surfaces that are too rough have surface peaks that will damage the gear teeth as they interact. Gears with surfaces that are too smooth, for example below 3 micro-inches Ra, cannot retain sufficient lubrication between adjacent teeth, resulting in an increased tooth wear rate.
Consequently, there is a need for gears having properly shaped teeth with improved surface finishes below approximately 10 micro-inches Ra in order to maximize the life and overall performance of gears.
The present invention is directed to a gear having a surface finish between approximately 5 micro-inches to approximately 10 micro-inches Ra for improved contact fatigue life, improved wear resistance, reduced friction and improved gear performance.
Most gears are manufactured using various gear cutting and shaping techniques, including hobbing and shaving, which result in surface roughness greater than 15 micro-inches Ra. Numerous methods exist for polishing metal surfaces in order to get a reduced surface roughness, including chemically accelerated vibratory polishing, electrochemical polishing, and mechanical polishing. When surface roughness is reduced to between approximately 5 micro-inches Ra to 10 micro-inches Ra, the maximum contact stress can be reduced more than fifty percent (50%). Similarly, the subsurface shear stress can be possibly reduced by approximately thirty percent (30%) to fifty percent (50%). The reduced contact stress and reduced shear stress results from improved lubrication conditions between the gear teeth. That is, the smoother the gear surface, and therefore the lower the roughness, the higher the film thickness ratio, xcex, and the greater the overall lubrication. As the film thickness ratio is increased, the lubrication is better, therefore the friction is lower, and the surfaces are better separated with a layer of lubricant. As a result of both surfaces not having at most limited direct peak contact, the contact pressure and the subsurface stresses are reduced. Also, the heat generation and the temperature rise in the gear tooth contact will be reduced due to the fact that direct rough surface peak contact and robbing are the main cause of high friction and heat generation. Reduce operating temperature and better controlled thermal equilibrium condition would greatly improve customer satisfaction, increase life and reduce warranty cost for geared products such as transmissions. It will also reduce energy consumption and improve efficiency.
It is also possible, however, to have a gear that is too smooth. For example, a gear with a surface roughness of approximately 1 micro-inch Ra to approximately 3 micro-inch Ra is too smooth, resulting in reduced oil retention for lubrication. This is because the lubricant requires some degree of surface roughness from which to adhere.